The present invention relates to a catalyst containing catalyst gauzes made of noble metal. Such noble-metal catalysts are used, in particular, for gas reactions, such as in the manufacture of hydrocyanic acid according to the Andrussow process and in the manufacture of nitric acid according to the Ostwald process. In order to provide large catalytically active surfaces for these reactions, such catalysts usually have spatial gas-permeable structures.
Often, these noble-metal catalysts consist of catalyst gauzes which are designed, for example, in the form of weft- or warp-knitted or woven fabrics made of noble-metal wire. In addition to the bending strength, the ultimate tensile strength, and the ductility of the noble-metal wires, the wire diameter also plays a role in limiting the geometric shape of the catalyst gauze. Noble-metal wires having diameters ranging from 50 to 120 μm and ultimate tensile strengths ranging from 900 to 1050 N/mm2 are the only ones suitable, for example, for knitting wires of specific platinum-rhodium, platinum-palladium-rhodium, palladium-nickel, palladium-copper and palladium-nickel-copper alloys.
As a result of these properties of noble-metal wires, other structural catalyst properties, such as the catalyst mass per surface unit and the number of meshes per surface unit, are also defined within certain limits. For this reason, the reaction process to be catalyzed may only be optimized to a limited extent by means of these structural catalyst properties.
To provide for better adjustment of these structural catalyst properties in known catalyst gauzes, U.S. Pat. No. 5,669,680 proposes incorporating a plurality of noble-metal wires having a helical, spiral-like structure into the meshes during the manufacture of the catalyst gauzes. Such a process extends the planar two-dimensional gauze into the third spatial dimension. These gauzes are, therefore, also referred to as “three-dimensional gauzes.” Due to the helical structure, it is, for example, possible to manipulate both the active catalyst surface and the mass of the catalyst per surface unit via the thickness of the incorporated wire or via the number of spirals of the helical structure. The incorporated wires also facilitate the manufacture of heavy three-dimensional gauze systems. As a general rule, the incorporated noble-metal wires extend linearly and in parallel to each other, thus also giving the catalyst gauze a preferential direction that is defined by their extension.
Usually, a plurality of catalyst gauzes are installed in the reaction zone of a flow reactor, and are often arranged in series. A flow reactor for catalytic oxidation of ammonia in which the catalyst is designed as a packing of a plurality of catalyst gauzes that extend in parallel to each other is known from DE 602 01 502 T2. The packing is arranged in the reaction zone in such a manner that the plane spread by the catalyst gauzes extends vertically to the flow direction of a fluid containing the reactants to be converted.
The arrangement of the catalyst packing extending transversely to the flow direction produces a flow resistance which, among other factors, depends on the porosity of the catalyst packing. While the period of application becomes longer, a decrease in porosity and an increase in flow resistance are often observed, whereby a uniform flow that is constant over time is prevented and a reproducible average product yield is impaired.